Measurement of the azimuthal anisotropy for charged particle production in √sNN = 2.76 TeV lead-lead collisions with the ATLAS detector

Abstract

Differential measurements of charged particle azimuthal anisotropy are presented for lead-lead collisions at √sNN = 2.76 TeV with the ATLAS detector at the LHC, based on an integrated luminosity of approximately 8 μb⁻¹. This anisotropy is characterized via a Fourier expansion of the distribution of charged particles in azimuthal angle relative to the reaction plane, with the coefficients vn denoting the magnitude of the anisotropy. Significant v₂–v₆ values are obtained as a function of transverse momentum (0.5 < pτ < 20 GeV), pseudorapidity (|η| <2.5), and centrality using an event plane method. The vn values for n≥3 are found to vary weakly with both η and centrality, and their pτ dependencies are found to follow an approximate scaling relation, v1/nn(pτ)∝v1/22(pτ), except in the top 5% most central collisions. A Fourier analysis of the charged particle pair distribution in relative azimuthal angle (Δφ=φa−φb) is performed to extract the coefficients vn,n=⟨cosnΔφ⟩. For pairs of charged particles with a large pseudorapidity gap (|Δη=ηa−ηb|>2) and one particle with pT<3 GeV, the v2,2–v6,6 values are found to factorize as vn,n(paT,pbT)≈vn(paT)vn(pbT) in central and midcentral events. Such factorization suggests that these values of v₂,₂–v₆,₆ are primarily attributable to the response of the created matter to the fluctuations in the geometry of the initial state. A detailed study shows that the v₁,₁(paτ,pbτ) data are consistent with the combined contributions from a rapidity-even v₁ and global momentum conservation. A two-component fit is used to extract the v₁ contribution. The extracted v₁ is observed to cross zero at pT≈1.0 GeV, reaches a maximum at 4–5 GeV with a value comparable to that for v₃, and decreases at higher pτ.